NFATc1, NF?B, cFOS, PU.1/SPI1 and MITF are transcription factors (TFs) essential for osteoclast differentiation from myeloid precursors. However, large gaps still remain in our understanding of the interplay between these TFs and how this interplay contributes to osteoclast differentiation and function. Importantly, critical questions about how the dynamics of TF regulation contributes to clinical outcome of human bone related diseases are unresolved. During the last cycle of this grant we used newly available functional genomic approaches and new bioinformatics approaches to develop a hierarchical network model that explains the interactions between these factors and their relative roles in osteoclast differentiation. Based on this model our overriding hypothesis is that PU.1 and MITF are at the apex of an osteoclast transcription factor network in osteoclasts and their myeloid precursors that initiates and maintains the differentiated state in response to signals received from the local microenvironment. In the current proposal, we aim to test this model using both in vivo and in vitro experiments and importantly to extend these studies into normal human osteoclasts and circulating osteoclast precursors from rheumatoid arthritis patients. Further we will test the importance of this network and associated factors in a preclinical mouse model for rheumatoid arthritis, using approaches that genetically target Pu.1 or use small molecule inhibitors to target PU.1/MITF-dependent epigenetic changes in pre-osteoclasts. By combining our efforts and expertise, our multidisciplinary team will focus on the task of understanding the mechanisms that govern osteoclast differentiation and function, and the relevance of these factors in human osteoclasts.